The present invention relates to a TDMA (Time Division Multi-Access) system and, in particular, to a local oscillating frequency synthesizer of TDMA system for monitoring by idle slots.
In a local oscillating portion of the mobile communication terminal, which is used in the TDMA system, as a high speed switching frequency synthesizer for, subsequent to a transmission slot and reception slots, switching the frequency to observe the electric field and BER (Bit Error Ratio) of other adjacent station, a direct digital synthesizer have conventionally been used.
FIG. 1 is block diagram showing a prior art frequency synthesizer using a direct digital synthesizer 27. The frequency synthesizer includes a PLL (Phase-Locked Loop) frequency synthesizer comprising a voltage control oscillator (VCO) 19, a buffer amplifier 20, a fixed frequency divider 21, a phase comparator 22, a charge pump (CP) 23 and a low-pass filter (LPF) 24. The frequency synthesizer further comprises a low-pass filter (LPF) 25, a D/A (Digital-to-Analog) converter 26 and a reference oscillator 28.
In the direct digital synthesizer 27, a reference frequency is digitally generated by using an oscillating frequency from the reference oscillator 28, and after passed through the D/A converter 26 and the LPF 25, it is entered to the phase comparator 22 of the PLL synthesizer. On the other hand, the output frequency of the voltage control oscillator (VCO) 19 is divided by the fixed frequency divider 21 to be entered to the phase comparator 22 where it is compared in phase with the reference frequency. The resulting error signal is converted into a voltage corresponding to the phase difference between the compared frequencies by the charge pump (C.P.) 23 and the LPF 24 to control the oscillating frequency of the voltage control oscillator 22 to generate a carrier.
Communication channel (CH) designation data 4 are entered to the direct digital synthesizer 27 to change the reference frequency. As a result, even if the channels are switched at an interval of 25 KHz, the compared frequency at the phase comparator 22 can be set high to allow a high speed frequency switching.
FIG. 2 illustrates how the slots are disposed in the TDMA system, in which it is possible to read the monitoring information on the electric field level of the other base station by utilizing an idle period 31 of time other than the reception slot 29 and the transmission slot 30. In this case, the following will become necessary: by entering channel designation data 5 of the other station to be monitored to the direct digital synthesizer 27, subsequent to the transmission and reception slots, the oscillating frequency 32 is switched to other station's channel designated by the data 5 to monitor the electric field, BER and the like on the other station's channel, and at the next transmission and reception slots, the oscillating frequency is returned to the original communication channel. To this end, the above-described high speed frequency switching technique has been used.
In such a local oscillating frequency synthesizer using the conventional direct digital synthesizer as discussed above, in order to digitally generate the reference oscillating waveform, it is necessary to use the digital frequency generating portion and the D/A converter portion, which consumes the electric power greatly.